The major global event that began in 1939 with the invasion of Poland was World War II. This conflict erupted when Nazi Germany, under Adolf Hitler, invaded Poland on September 1, 1939, prompting Britain and France to declare war against Germany. World War II became the deadliest and most widespread conflict in history, involving numerous countries and resulting in significant military and civilian casualties. It was characterized by major battles, the Holocaust, the use of atomic weapons, and ultimately led to the defeat of the Axis powers in 1945. The war reshaped international relations, leading to the Cold War and the establishment of the United Nations.